The present invention relates generally to machine vision equipment, and more particularly, relates to an apparatus and method for temperature compensation of three-dimensional measurements from a non-contact sensor in a workpiece manufacturing station.
Demand for higher quality has pressed manufacturers of mass produced articles, such as automotive vehicles, to employ automated manufacturing technology to assemble, weld, finish, gauge and test manufactured articles. Machine vision is a key part of today's manufacturing environment. Machine vision systems are used with robotics and as in-process gauging equipment to monitor and improve the manufacturing process and thereby improve quality and reduce cost of the articles produced.
In a typical manufacturing environment, there may be a plurality of different non-contact sensors, such as optical sensors, positioned at various predetermined locations within the workpiece manufacturing, gauging or testing station. The workpiece is placed at a predetermined, fixed location within the station, allowing various predetermined features of the workpiece to be examined by the sensors. Preferably, all of the sensors are properly positioned and carefully calibrated with respect to a fixed frame of reference, such as a common reference on the workpiece or at the workstation.
Achieving high quality manufactured parts requires highly accurate, precisely calibrated machine vision sensors. Not only must a sensor have a suitable resolution to discern a manufactured feature of interest, the sensor must accurately measure with respect to an external frame of reference so that relevant data regarding the manufactured parts can be reported.
One area of concern with sensor accuracy is measurement variations caused by temperature changes at the workpiece manufacturing station. Typically, the entire manufacturing assembly facility will experience significant fluctuations in temperature throughout the workday. As the temperature changes, the entire workpiece manufacturing station changes, including each of its various components. It may be possible to model the response of each component of a workstation with respect to changes in temperature. For instance, it is known that all of the components that comprise the inspection station: the workpiece, the tooling that transports and secures the workpiece in the station, the sensor mounting structure, the sensor mounting hardware, and the sensors, will expand and contract with temperature variations and that these physical changes in the components of the workstation will also cause deviations in the resulting measurements. Consequently, the enormity of the task of accurately modeling each of the relevant components in the typical manufacturing workstation can be readily appreciated.
Therefore, rather than attempting to empirically develop suitable compensation data to correct the response of each workstation component, a temperature compensation system of the present invention employs a system approach. Each of the components of the workstation that may be affected by variations in temperature are viewed as a system. A characteristic curve is determined that represents resulting variations in a sensor's measurements caused by changes in temperature. In this way, the characteristic curve incorporates all of the variables in the system that may affect a measurement from that particular sensor and thus provides an accurate means for providing temperature compensation for the sensor measurements.